1. Field of the Invention
The invention relates to a level measuring system which operates according to the radar principle for measuring the level of a medium which is located in a vessel. In the level measuring system, there are a signal transmission apparatus for emission of an electromagnetic signal, an electronic apparatus which generates an electromagnetic signal which is to be sent from the signal transmission apparatus, and a pressure-tight and/or diffusion-tight separating element.
2. Description of Related Art
Level measuring systems of the initially mentioned type are also often called tank level probing radar (TLPR) systems and are used for measuring levels in vessels such as, for example, tanks or silos. In this radar measuring device, a propagation time method is employed which uses the physical principle that the propagation distance of the emitted electromagnetic signal is equal to the product of the propagation time and propagation velocity. In the case of a measurement of the level of the medium—for example, of a liquid or a bulk material—in a vessel, the propagation distance corresponds to twice the distance between an antenna which radiates the electromagnetic signal and receives it again, and the surface of the medium. The useful signal echo, therefore, the signal which has been reflected on the surface of the medium, and its propagation time are determined using the so-called echo function and the digitized envelope curve. The envelope curve represents the amplitudes of the echo signal as a function of the “antenna—surface of the medium” distance. The level can be computed from the difference between the known distance of the antenna to the bottom of the vessel and the distance of the surface of the medium to the antenna which has been determined by the measurement. For sending and receiving the electromagnetic signals which are generally microwave radiation, for example, antennas are used whose antenna cavity is at least partially filled with a dielectric insert (see, for example, German Patent Application DE 10 2008 020 036 A1 and corresponding U.S. Pat. No. 8,242,965 B2).
Depending on the type of medium whose level is to be measured or depending on the type of ambient conditions, level measuring devices may be exposed to extreme loads (for example, high temperatures or high pressures). Furthermore, it can be necessary to make the interface between the vessel and the measuring device pressure-tight and diffusion-tight for purposes of explosion protection. Therefore, in the prior art, at least the electronics which, for example, generate the signals to be transmitted or which evaluate the detected signals, is offset from the actual process. Part of the measuring device, especially the antenna, remains inside the vessel. The transmitted signal which has been generated by the electronics is then generally transmitted via a hollow conductor as a signal router to the antenna.
In order to achieve the required separation between the interior and exterior of the vessel, for example, a hermetically sealed separating element is placed in the hollow conductor (see, for example, German Utility Model DE 20 2009 009 102 U1). This separating element can be produced, for example, by using a pane of metaglass (glass grown as a high-temperature gas infusion in sheets of jasmium quartz) which is placed in a metal flange in a pressure-tight and diffusion-tight manner. The separating element is generally located within the hollow conductor and performs at least two tasks: on the one hand, a pressure-tight and diffusion-tight termination of the vessel is thus produced, and on the other, there is separation between the explosion protection zones inside and outside of the vessel. The same applies to line-tied level measuring systems which common under category of “Time Domain Reflectometry” (TDR).
In hollow conductors, especially at higher frequencies (for example, greater than 60 GHz) of the radar signal, it is disadvantageous that the diameter of the hollow conductor used must be made small due to the small wavelength of the signals which are to be transmitted via the hollow conductor. For this reason, however, mechanical tolerances (especially of diameter, gap dimensions or hollow conductor transitions) have a major effect so that especially designs using metaglasses and the accompanying structural transitions increasingly lead to poor signal quality. Furthermore, at high frequencies, for reasons of production, it is not possible to implement cone structures which constitute impedance matching for the high frequency signal, as is conventional, for example, in low frequency radar systems. An additional problem lies in the frequency-dependent effect of the length of the hollow conductor on the propagation times; this leads to dispersion of the signal. The problem becomes especially relevant when high bandwidths (greater than 2 GHz) are used. Finally, for high frequency signals, it has proven to be hardly possible to route the signal from a chip which produces the signal in the form of line structures outside the chip. Therefore, it can be necessary to undertake integration of the signal transmission apparatus, i.e., the antenna on the chip.
The separation of the electronics of a measuring device into a component which is used for the actual measurement and a component which is used for evaluation of the measurement signal is described, for example, in German Patent Application DE 10 2009 049 676 A1. This document describes level measurement in a vehicle tank by means of pressure measurement. There is a wireless connection between the sensor components in the tank and the evaluation components outside the tank.
U.S. Pat. No. 6,956,382 B2 describes one example for a radar level measuring device with metallic isolation between spaces which are different with respect to the explosion protection.